Searching for Far-ultraviolet Auroral/Dayglow Emission from HD 209458b
Abstract
We present recent observations from the Hubble Space Telescope-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD 209458b. We obtained medium-resolution (R ~ 20,000) far-ultraviolet (1150-1700 Å) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8σ) feature at +15(± 20) km s-1 in the rest frame of the planet, at λ1582 Å. This feature is consistent with emission from H2 B-X (2-9) P(4) (λrest = 1581.11 Å) however, the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short-period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2010
- DOI:
- 10.1088/0004-637X/712/2/1277
- arXiv:
- arXiv:1002.3218
- Bibcode:
- 2010ApJ...712.1277F
- Keywords:
-
- planetary systems;
- stars: atmospheres;
- stars: individual: HD 209458;
- ultraviolet: stars;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted to ApJ. 12 pages, 5 figures, 4 tables